Prior sound reproduction systems attempted to reproduce the audio realism of three-dimensional spaciousness through the application of sophisticated electronic technology to virtualize the third dimension of sounds or to form immersive sound fields with multichannel surround sound formats. However, this can be costly with limited effectiveness, and the realism of three-dimensionality of sound is left partly to the listener's imagination as to what is being heard and what is intended to be heard.
Multi-speaker formats, such as a 5.1 speaker set-up, are well known for creating an immersive sound space. However, the use of large numbers of speakers adds to the “confusion” of the total sound space. For a listener, increasing the number of speakers increases the number of segments of two-dimensional lateral sound fields. This condition is prone to producing poor and confusing stereo effects and less sound transparency. Some systems attempt to broaden the soundstage by processing front-channel signals to fool the ear-brain into “hearing” sounds beyond the left and right speakers. However, to experience the optimal effect of such systems, the listener is often required to be confined to a particular seating position within a small sweet spot formed within the sound space. As one moves from the sweet spot for listening to the soundscape produced by the multiple speakers, the virtual surround effect collapses. This can happen even when one merely rotates one's head. There may also be smeared images where different frequencies can appear to come from different directions.
Some sound reproduction systems record and playback binaural sound. Binaural recording is a method of recording audio which uses a special microphone arrangement. Binaural recording is done with an artificial or “dummy” head replicating the human head, and small omnidirectional microphone condensers mounted at or near the entrance to the ear canals in the artificial head. Typical stereo recordings, on the other hand, are mixed for loudspeaker arrangements, and do not factor in natural crossfeed or sonic shaping of the head and ear.
People perceive sound in three dimensions, and localization of sound depends on how the sound waves from the same source differ from each other as they reach the left and right ears. A Head Related Transfer Function (HRTF) describes how free-field incoming sound waves are modified by the presence of a listener in the field, including the scattering of sound off the listener's pinna, head, and torso. A HRTF is the Fourier transform of the impulse response from the source of the sound to the human tympanic membrane (eardrum). For example, to generate a sound that seems to come from the right side of the ear, we need to have the HRTF of the human ear's impulse response to sound coming from the right direction. Since the HRTF is from the source of the sound to the tympanic membrane (eardrum), it is a function of frequency, azimuth and elevation (the path that sounds travel to the ear), as well as the pinna structure (where sound is collected and reflected into the eardrum).
HRTFs can be used to generate binaural sound. If properly measured and implemented, HRTFs can generate a “virtual acoustic environment.” Measuring HRTFs, however, can be expensive. A typical set up requires an anechoic chamber and high quality audio equipment. The anechoic chamber is used to minimize the influence of early reflections and reverberation on the measured response. Even the most carefully taken measurements, however, suffer from what often is referred to as “cones of confusion” and “inside the head” effects. And HRTFs can show considerable person-to-person variability. For the mass market, there have been attempts to use generic HRTFs, but these do not work as well as individualized HRTFs.
Present formats of multichannel surround sound, virtualization, binaural and others designed to overcome these shortcomings of depthless sound field are problematic. In order to repropagate a three-dimensional sound space which approximates the original pre-recorded sonic state, the experiments indicate that instead of relying on the powers and versatilities of electronic audio and digital processing and present techniques of recording and remixing, a unique approach to resolve this problem is necessary. There is a need for a method which can remedy the deficiency mentioned above by recreating in phantom form the three-dimensionality of multichannel recordings when replayed on conventional electronic playback systems.